In a toroidal continuously variable transmission for a vehicle such as that disclosed in U.S. Pat. No. 5,542,890, there is a tendency for the real drive ratio to diverge from the target drive ratio depending on the direction or size of transmitted torque, due to slackness of the bearings supporting the power rollers or the elastic deformation of the trunnions. Specifically, a deviation towards upshift tends to occur when the input torque is decreasing and a deviation towards downshift tends to occur when the input torque is increasing. These deviations, that is to say, the drive ratio error is large when the input torque is large.
In this context, Tokkai Hei 8-338490 published in 1996 by the Japanese Patent Office discloses the feedback correction of drive ratio error on the basis of the correction value TS1 calculated from a target drive ratio RTO and an input torque Tin to the transmission.
The target torque RTO is obtained from the vehicle speed VSP and throttle opening TVO of the engine.
However this type of drive ratio control creates the following problems when applied to vehicles provided with a traction control device which accompanies braking.
A traction control device which accompanies braking decreases the engine output corresponding to the slip of the drive wheel and operates braking of the vehicle wheels so that grip is not lost with respect to the road surface. The basis of such a device lies in the application of the braking if drive wheel slip is detected even when the vehicle is accelerating due to depression of the accelerator pedal.
Hence for example if traction control is applied during acceleration and the braking is applied, the rotation speed of the drive wheels will temporarily decrease. As a result, the toroidal continuously variable transmission will temporarily vary the drive ratio in the direction of downshift in a similar way to when there has been an increase in engine torque.
Since this is not a result of an actual loss of vehicle speed, the variation of the drive ratio is either totally unnecessary or leads to so called hunting in which the drive ratio oscillates until it finally falls to an appropriate value.